MJO Research at PSD:

An MJO Primer: What is an MJO and how does it affect the weather?

The Madden-Julian Oscillation is a mode of sub-seasonal
atmospheric variability that influences the location and strength of tropical
precipitation as in this classic schematic (left)

from Rol
Madden and Paul Julian. Typically the convectively active stage of an MJO
starts over the equatorial Indian Ocean and moves slowly eastward at 3-5
m/s toward the west and central Pacific Ocean. The active stage is followed
by a convectively suppressed stage and together they give rise to precipitation
anomalies (i.e., departures from normal) that have a "dipole" structure.
The "cycle" repeats itself approximately every 5 days. Sometimes
the debris from a previous event starts a new MJO over the Indian Ocean.
A composite OLR
animation illustrates the typical sequence of precipitation anomalies
during an MJO (courtesy of Adrian
Matthews).

The MJO was first described in 1971 but a field project
held over the west equatorial Pacific during 1992-93 raised awareness of
the MJO as a coherent phenomenon, possibly useful for weekly predictions
of tropical precipitation and extratropical weather patterns. The
MJO is best defined over the oceanic warm pool, which extends from the
Indian Ocean to the central Pacific and where complex variations of precipitation
occur from day to day. The warm pool is generally
defined by sea surface temperatures that are > 28C. As
a result of this intense activity, the MJO is sometimes difficult to see
in a sequence of satellite pictures of the Tropics. This
link shows a satellite animation of a strong MJO that occurred during
December 2003 to January 2004. Note the non-steady or transient nature of the convection,
some of which can be seen more clearly when the data are averaged in space
or time. Ways to monitor the MJO are illustrated later.

b)How can an MJO affect the
weather?

When an MJO moves eastward through the Indo-Pacific
Ocean region, it produces large, slowly changing departures from the normal
tropical precipitation. These departures change the
atmospheric circulation and, through the phenomenon of Rossby wave dispersion,
the changes can propagate into higher latitudes. The
accompanying picture

Rossby Wave Propagation

from Sardeshmukh and Hoskins illustrates the phenomenon
for the situation where the "base state" atmospheric circulation is extremely
simple. The picture shows upper atmospheric ridges and troughs emanating
from a region of "precipitation" forcing over the west Pacific and extending
poleward into both hemispheres. The circulation features can alter the
path and intensity of synoptic waves in mid-latitudes and thereby can affect
the weather. The actual "base state" through which
these Rossby waves propagate is very complicated, involving jet streams,
storm tracks and other regional circulation features. These
features channel or interact with the Rossby wave energy and result in
a pattern of troughs and ridges that is highly variable from case to case. This
makes it difficult to predict the effect of an MJO, especially in regions
far removed (e.g., North America) from the MJO's centers of action over
the oceanic warm pool. The MJO signal is fairly small
north of 30N on average.

c) How do MJOs interact
with slower climate processes like ENSO?

MJOs may play a role in the transition to an El Nino
or La Nina.This means they help determine details
of the timing and/or amplitude of a warming or cooling event. However,
the transitional stage of an ENSO event is complicated and the MJO's role
is still being investigated. MJOs were prominent
during the 1996-97 northern winter at the early stage of the 1997-98 El
Nino and this led to heightened awareness of a possible MJO-ENSO link.The
1996-97 MJO activity was by some measures the greatest in the ~30 year
record of outgoing longwave radiation (OLR, a proxy for deep tropical convection). On
the other hand, 1997-98 had the weakest activity in the record.The two
years are contrasted here using a time-longitude or Hovmoller diagram of
total OLR for 1996-97 and 1997-98

1996-97 OLR

1997-98 OLR

Once SST anomalies associated with an ENSO are in
place, the MJO life cycle is influenced by the SST anomalies. When or if
an MJO develops, the simplest effect during El Nino is a farther eastward
movement of the convection into the central Pacific whereas during La Nina
the convection anomalies barely get into the western Pacific. In both cases
the MJO still "starts" over the Indian Ocean.

Despite this influence on the MJO life cycle, it is
unclear whether overall MJO activity is influenced by the phase of the
ENSO cycle. The natural variation of MJO activity is too large and observed
datasets are too short to provide a definitive answer to this question.
Transient convection at all time scales increases when sea surface temperatures
(SST) reach ~ 29C (84F). On the other hand, there is a significant relationship
between MJO activity and SST anomalies over the western Pacific Ocean (140-180E).

Scatter
plot of SST vs. MJO activity

When ENSO is in a neutral stage during
the northern fall season and anomalously warm SST are present in the west
Pacific Ocean, stronger MJO activity follows during the winter season.
The 2003-04 northern winter was a good illustration of this relationship.

d) How can the atmosphere and ocean signals associated with the MJO
be determined?

The MJO life cycle can be broken down into stages using an index based,
for example, on the location of the MJO's tropical convection anomaly.
The atmosphere's observed large-scale circulation and/or local weather
anomalies can then be averaged over many cases to produce a "composite"
`anomaly for each stage. Time filtering is usually applied to exclude higher
and lower frequency variability. The results produce an estimate of the
MJO signal or influence based on the observational record. The composite
anomalies of circulation and weather are generally weak in the extratropics
and moderately strong in the tropics and subtropics. They will be described
in more detail later.

e) How do MJOs interact
with faster weather processes like synoptic scale waves and wavetrains?

The composite anomalies, assuming they are statistically
significant and large enough, produce persistent (1-3 week) changes in
the atmospheric flow due to the MJO. These changes can influence the development
and propagation of synoptic-scale weather systems, i.e. they influence
the storm track. For example, during one stage Pacific Ocean storms tend
to be stronger and farther south when they make landfall on the U.S. west
coast. At the opposite stage the storm or wave energy may split and move
south into the tropics and north into Canada, favoring storms over the
central U.S. Plains. Because the MJO extratropical signal is weak there
are large variations of the actual circulation or weather observed in individual
cases. Other processes may overwhelm or mask the
MJO signal.

Daily monitoring of many individual cases has produced
qualitative evidence for interaction between the circulation induced by
flare-ups of convection within the MJO's convective envelope and synoptic
scale waves or wavetrains passing by in mid-latitudes and the subtropics. These
daily interactions are large amplitude and sometimes contribute to the
rapid initiation of the composite MJO signal and/or major transitions in
weather patterns.Until GCMs are able to simulate
MJOs, we rely on daily monitoring and a subseasonal synoptic model to provide
an early indication of such situations.

f) How is the MJO signal
extracted from climate forecasts?

This section will discuss how the MJO signal is pulled from models
and forecasts. Ideally, the MJO signal in the forecasts in the group will
be displayed here. Advatanges and disadvantages of the different methods
will be discussed.

Project on EOFs

Modes of variability are extracted from the data which describe
the MJO. Then, the forecasted time series of a variable like velocity potential
is projected onto those modes and an amplitude and phase is extracted.